The influence of the Si-H2 bond on light-induced degradation and the thermal recovery of a-Si films and a-Si solar cells were studied. The influence of the Si-H2 bond on light-induced degradation depends on the impurity content in a-Si films, and light-induced degradation can be reduced by decreasing the Si-H2 bond density in a-Si films with impurity content of 1018 cm-3. The activation energy of the thermal recovery process was about 1.0 eV, and it did not depend on the Si-H2 bond density. However, an irreversible phenomenon was observed in film properties and solar cell characteristics with high Si-H2 bond density. It is thought that the structural flexibility of the Si-H2 bond is related to this irreversible phenomenon.
The hydrogen plasma post-treatment of hydrogenated amorphous silicon (a-Si:H) has been investigated to obtain high-quality wide-gap films. The hydrogen plasma treatment after film deposition substantially increases the hydrogen content and the optical gap of a-Si films without deteriorating their optoelectric properties within the range of treatment conditions in this study, where no microcrystallization of the films is observed. A photoconductivity of ∼10-5 Ω-1 cm-1 and a photosensitivity (the ratio of photoconductivity to dark conductivity) of >106 are obtained for a-Si:H films with an optical gap of >1.7 eV from the (αhν)1/3 plot (>2.0 eV from Tauc's plot) under AM-1, 100 mW/cm2 illumination. An extremely high open circuit voltage of >1 V is obtained for an a-Si single-junction cell whose i-layer was fabricated using the hydrogen plasma treatment.
An rf plasma decomposition of SiH4 under a magnetic field was investigated. It was confirmed by the optical emission spectra that a high-electron-density plasma can be produced under a magnetic field. High-quality a-Si films with a photosensitivity of σph/σd of 7×105 were obtained at a high deposition rate of 10 Å/s under the magnetic field. The a-Si solar cells with i-layers deposited at a high deposition rate under a magnetic field have a higher open-circuit voltage and a higher conversion efficiency than those without the magnetic field; a conversion efficiency of 10.1% under AM1(100mW/cm2) illumination was obtained at a deposition rate of 10 Å/s. The rf plasma decomposition of SiH4 under a magnetic field is thought to be very suitable for fabricating a-Si solar cells with a high conversion efficiency at a high deposition rate.
Transient light-induced electron spin resonance (LESR) at 120 K, has been used to investigate potential differences between intrinsic (stable) and light-induced (metastable) defects in a-Si:H through changes in the line shape. Previously, we have reported that when the line shape is decomposed into broad and narrow components, the narrow component decreases dramatically, relative to the broad component, with increasing light-soaking time. The present results indicate, however, that similar changes are not observed when the defect density is increased by changing deposition conditions or by high-temperature annealing. A dangling-bond-conversion process involving charged dangling bonds is proposed to explain these changes. We suggest that stable and metastable defects play different roles in transient LESR and may occupy different energy positions in the gap of a-Si:H.
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